Re: [RTnet-developers] multicast support

[Jan K. <ki...@rt...>]

Amine El hedadi wrote:
> Linux use timer in muticast to excute periodically a function to keep session 
> between switch & computer whitch need multicast flux , so it use one timer by 
> multicast connection(IGMP protocol). this work not need a real time 
> behaviurs, so why I have asked about using the same timer in RTNET framework. 
> I can use  real periodic task but I don't need real time behaviors in this 
> case, may be  rt task with low priority.
> 

I see, my knowledge about multicast has just been proven to be rather 
limited :).

The point with a Linux timer is not that it is too unreliable for this 
task (BTW, about what period dimensions are we talking here?), the 
problem is that you cannot issue a packet from non-RT Linux context 
(IRQ, tasklet, process, whatever). So you need at least a low prio 
real-time task (one step above Linux, see how RTcfg works WRT timers 
e.g.) to transmit this keep-alive stuff.

I guess you have to transmit those packets only if no other traffic was 
present on the specific multicast channel (MAC address), haven't you? 
Isn't it possible to code this feature into the application? I think it 
knows best when it last received or transmitted some data. Or am I just 
missing some important characteristics of the IGMP protocol?

In any case, I think the IGMP part, which seems to be required when it 
comes to IP multicasting, is part of step 2, while the first step should 
be to get the low level multicast support (Ethernet layer) working. Or 
does this appear to be too tightly coupled for you?

Jan